This invention relates to a protection system for preventing second breakdown destruction of bipolar transistors, in a voltage source transistor inverter, in the event of a shootthrough fault or an output short circuit.
In a typical voltage source transistor inverter at least two pairs of power bipolar transistors are series-connected across the d-c bus over which a d-c voltage is received from a d-c voltage source. The circuit junction of each pair of transistors connects to a load, such as an induction motor. By switching the transistors on and off (namely, between saturation and cutoff) in a predetermined sequence the d-c voltage is effectively converted to a-c voltage for application to the load. For example, when the inverter includes three pairs of bipolar transistors (which may be power darlingtons) the inverter output voltage will exhibit a six-step waveshape to approximate a sine wave.
Under normal conditions, a series-connected pair of transistors will never be turned on at the same time by the control circuitry for the inverter. Unfortunately, however, a transistor can be inadvertently switched on, such as by noise, when it should be non-conductive, and if the inadvertently triggered transistor becomes conductive at the same time that the other transistor in the pair has already been turned on by the control circuitry, essentially a short circuit will be created across the d-c bus through the emitter-collector conduction paths of the two faulted transistors. The shunt-connected filter capacitor of the d-c voltage source thereupon discharges and, in the absence of some protection arrangement, would destroy at least one of the two transistors within a few microseconds due to the transistor load line entering the area of second breakdown. This is commonly referred to as a "shootthrough fault."
To appreciate the magnitude of the fault current, in a 20 horsepower inverter drive, for example, the filter capacitor (which may actually comprise a series of separate parallel-connected capacitors) may typically have a capacitance of 13,200 microfarads and the d-c voltage on the d-c bus, and therefore across the filter capacitor, may be around 300 volts. If there is a shootthrough fault which short circuits the d-c bus, a peak fault current of up to 10,000 amperes could flow through the two conducting faulted transistors, the fault current being limited only by the filter capacitor's effective series resistance. Moreover, since the filter capacitor can deliver more current to the transistors than they can accept, the d-c bus voltage will remain at its normal high level (300 volts) at the beginning of a shootthrough fault. As a result, the full bus voltage will be across the emitter-collector conduction path of the inadvertently triggered transistor at a time when its collector current (namely, the fault current) is of very high amplitude. The transistor load line will therefore fall well within the second breakdown area and the very high peak power (the collector voltage V.sub.ce multiplied by the collector current I.sub.c) will destroy the transistor.
To overcome this problem, shootthrough fault protection schemes have been developed. A well-known prior arrangement provides a crowbar circuit, consisting of an SCR, across the d-c bus. When a shootthrough fault occurs, the SCR is fired into conduction to divert the fault current from the transistors to the SCR which has considerably better surge characteristics. However, since the SCR has a higher voltage drop (greater than one volt) than the two series transistors (0.3 volt+0.3 volt or 0.6 volt) some of the fault current still passes through the transistors and they are not completely protected. Also since the SCR must discharge the filter capacitor it has to have a large I.sup.2 T rating.
The shootthrough fault protection system of the present invention constitutes a significant improvement over those previously developed, especially over the prior arrangement described hereinbefore, achieving much greater protection of the transistors and yet being considerably simpler and less expensive in construction.